I have a ax15 with the internal slave cylinder and was wondering if that bell housing would fit on a nv3550. The ax15 I have is making a knocking noise in 5th gear, is it still safe to drive or driving it damage the rest of the trans?

All of the NV trans have external slaves So you would have to go that route as well, since the nose cone would need to be different on the 3550 for an internal slave to work.

And if 5th gear has a rattling sound, the bearing is probably out on that shaft. Continuing to drive it like that will most likely destroy the entire trans. If 1 of the rollers comes out of the bearing and goes bouncing around inside the trans case, I'm sure it won't end up very pretty in there.